Deyu Li
Vanderbilt University – Nashville, Tennessee, USA
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Phonon polaritons (PhPs) have attracted extensive attention in the study of light-matter interactions because of their origin as the hybrid modes generated from coupling between infrared photons and optically active phonons. In terms of energy transport, while the contribution of surface phonon polaritons (SPhPs) to near-field radiation has been well recognized, their roles in heat conduction along nanostructures have been studied only recently. In fact, because of the much lower number density of PhPs as compared to acoustic phonons, the PhP contribution to thermal conductivity is often limited with measured values lower than 0.5 W/m⋅K.
Boron nitride belongs to a class of highly interesting materials in nanophotonics with fascinating optical properties because of their hyperbolic polariton dispersions with multiple PhP branches in the Reststrahlen bands. Here we report on our recent discovery of strong thermal effects produced by non-equilibrium PhPs in boron nitride nanotubes (BNNTs).
To reduce the contact thermal resistance (RC) between BNNTs as nanofillers for thermally conductive polymer composites, we introduced a thin Au interlayer to eliminate the resistance related to phonon back reflection. While indeed, the Au interlayer results in much lower contact thermal resistance between BNNTs at elevated temperatures above 200 K, it is a surprise that the extracted contact thermal resistance with the Au interlayer becomes negative below 80 K for a 50 nm BNNT sample. This unexpected observation leads to the hypothesis that the Au coating could serve as an efficient launcher to introduce SPhPs propagating along the uncoated portions of the BNNTs, which alters the thermal circuits and yields a nominal negative RC. To verify this hypothesis, individual BNNTs with and without Au-coating at one end were measured to estimate the effects of PhPs in heat conduction along the uncoated tube segment. This leads to a remarkable PhP-mediated thermal conductivity of 9.5 W/m⋅K at room temperature, which is more than one order of magnitude higher than the values reported in the literature. Importantly, the PhP-mediated thermal conductivity increases with the sample length, suggesting ballistic PhP transport, which is corroborated by the long propagation lengths revealed in the near-field optical characterizations. Interestingly, the measured polariton thermal conductivity shows a clear upturn in the temperature range (430-470 K) corresponding to the Reststrahlen band of BNNTs, which represents the first experimental evidence of the strong thermal effects of the hyperbolic phonon polaritons of multiple branches.
Email: deyu.li@vanderbilt.edu